This invention relates to thermosetting epoxy resin powder coating compositions capable of producing a matt finish. Matt finish is defined herein as matt or semi-matt appearance of the cured film. Matt finish films have less than 30% gloss and semi-matt finish films have from 30 to 55% gloss as determined at 60.degree. C. by ASTM D 523.
Use of powder coating compositions is gaining interest in the coatings industry because of their positive environmental and economical aspects. Epoxy resin-based powder coating compositions generally produce glossy or semi-glossy coatings. However, for some applications such as coating a large surface, coatings producing a matt or semi-matt finish are preferred in order to avoid undesirable or unsuitable reflections of light.
U.S. Pat. Nos. 3,947,384 and 4,007,299 disclose the use of curing agents comprising a salt of a polycarboxylic acid having three or more carboxyl groups (such as trimellitic anhydride or pyromellitic acid) and an imidazoline derivative (such as 2-phenylimidazoline), in epoxy resin powder coating compositions which produce a matt finish. Said salts are prepared by dissolving the polycarboxylic acid in a polar aprotic solvent, such as dimethylformamide, dimethylacetamide or N-methylpyrrolidone, and then adding the imidazoline compound in the desired molar ratio, by itself or dissolved or suspended in the same solvent, under stirring at temperatures between room temperature and 100.degree. C. A period of one hour for completion of the salt formation is reported.
Japanese patent application 61,218,578 relates to the use of an imidazoline-isocyanuric acid adduct curing agent in epoxy resin powder coatings and discloses the preparation of the salt of isocyanuric acid and 2-undecylimidazoline by boiling both components in a molar ratio of 1:1 for 5 minutes in water and recovering the formed adduct.
The use of such adducts results in an epoxy-based powder coating which has low reactivity at relatively low cure temperatures.
It is an object of the invention to provide an epoxy resin powder coating composition which has a short cure time at relatively low temperatures and produces a matt or semi-matt finish.